Electronic monitoring devices are used by many government and private entities to detect and monitor the location of individuals wearing or associated with an electronic monitoring device. Electronic monitoring devices provide a cost effective solution to overcrowding of jails or prisons, and are often used to allow non-violent offenders to transition to society during a parole period, after being incarcerated for a period of time. Such devices can ensure confinement of the offender or monitored person to a particular location, such as at the offender's place of residence or at a rehabilitating institution, and may also determine whether the individual enters any places he is not allowed, also referred to as exclusion zones.
Electronic monitoring devices typically use either radio frequency (RF) communication with a second device, or Global Positioning System (GPS) technology to confirm the location of a monitored person, either in an absolute sense as with GPS, or relative to the second device which the electronic monitoring device is in RF communication with. GPS technology has certain limitations. For example, in some environments the GPS signal may be lost or blocked in or by tall buildings or other structures with layers of steel and concrete. It would be advantageous to know whether an interrupted signal, whether RF or GPS, is due to environmental or geographical reasons, or due to deliberate attempts by individuals to disable monitoring capabilities. For the purpose of knowing where the monitored person is, electronic monitoring devices are often designed to be attached to a monitored person for monitoring the movements or other activities of the person. Such electronic monitoring devices often include a tamper sensor for sensing tampering with the device or removal of the device from the person to whom the device was attached. The electronic monitoring device may then produce a tamper signal which is transmitted to an external receiver.
Other devices have been designed to detect tampering with the signal, such as placing metal shielding around an electronic monitoring device. Such metal shielding could prevent the device from being able to transmit RF signals to or receive RF or GPS signals from other devices. United States Patent Application 2010/0090825 describes incorporating a metal detection circuit into an electronic monitoring device, and combining output from such a circuit with other factors such as loss of GPS signal and the level of gain provided in an automatic gain circuit (AGC) in a GPS receiver to determine whether shielding is occurring.
United States Patent Application 2010/0090826 describes a different method of detecting shielding. This application describes the use of an auxiliary device in combination with an electronic monitoring device. The auxiliary device is disposed in close proximity to the electronic monitoring device, and emits an RF signal detected by the electronic monitoring device. If the electronic monitoring device fails to receive the signal from the auxiliary device, such a failure is considered to be indicative of shielding or some other type of tamper.
Even in light of these methods of detecting shielding, advancements in this area, and improvements in accuracy of shielding detection would be welcomed.